Returns Nothing if the argument is not a square,
Just r if r*r == n and r >= 0. Avoids the expensive calculation
of the square root if n is recognized as a non-square
before, prevents repeated calculation of the square root
if only the roots of perfect squares are needed.
Checks for negativity and isPossibleSquare.

Test whether a non-negative number may be a square.
Non-negativity is not checked, passing negative arguments may
cause any kind of havoc.

First the remainder modulo 256 is checked (that can be calculated
easily without division and eliminates about 82% of all numbers).
After that, the remainders modulo 9, 25, 7, 11 and 13 are tested
to eliminate altogether about 99.436% of all numbers.

This is the test used by exactSquareRoot. For large numbers,
the slower but more discriminating test isPossibleSqure2 is
faster.

Test whether a non-negative number may be a square.
Non-negativity is not checked, passing negative arguments may
cause any kind of havoc.

First the remainder modulo 256 is checked (that can be calculated
easily without division and eliminates about 82% of all numbers).
After that, the remainders modulo several small primes are tested
to eliminate altogether about 99.98954% of all numbers.

For smallish to medium sized numbers, this hardly performs better
than isPossibleSquare, which uses smaller arrays, but for large
numbers, where calculating the square root becomes more expensive,
it is much faster (if the vast majority of tested numbers aren't squares).